1. Field of the Invention
The invention pertains generally to the field of cathodo-luminescent phosphor materials and to cathode ray displays employing them and more particularly concerns improved penetration phosphors for use in bright color display cathode ray indicators.
2. Description of the Prior Art
In Applicant's copending application Ser. No. 505,710, entitled "Penetration Phosphors and Display Devices", and assigned to the assignee of the present invention, Applicant points out the particular requirements of cathode ray vacuum tubes for use in information displays such as air traffic control, radar, data processing and the like. In particular, Applicant points out that the particular requirements for such systems are generally not met by cathode ray tubes of the type conventionally available for color television viewing. For example, these particular systems generally do not require the many colors needed for a television screen; for example, the provision of blue may even be undesirable. While red and green are readily simultaneously focussed by the optics of the normal eye at the sensitive high resolution fovea, the blue component of the image is normally out of focus and its presence may even tire the eye. Further, the very center of the fovea is red and green sensitive, but blue is not appreciably sensed there. Since red and green light may purposely be mixed so as to yield the sensations of orange or of yellow in the eye, a wide range of colors adequate for information displays is available without the use of blue.
Further features are desirable in an information display; a major need is to provide brightness and contrast when the display is viewed in a high ambient light level, including sun light, whereas conventional color cathode ray television tubes are useful only at low or medium light levels. High resolution, especially in the center of the screen, not available in conventional tubes, is also desired. Relatively simple electron beam focussing and scanning elements are also desired so that either raster scanning or random beam positioning or both, as often required for presenting computer information, is easily achieved.
While several kinds of color television cathode ray tubes are currently available, including the older type with a mask with round holes, the in-line, slot-mask color tube, and the recent slit mask color tube, all of these use multiple guns and complex electron beam focussing and scanning arrangements and are generally not suited for use in information displays, especially where random deflection is needed. Resolution is poor, and sensitivity to stray and earth's magnetic fields is undesirably high. Because they require multiple cathode and multiple electrode systems, sensitivity to shock and to vibration is also excessive.
While originally conceived for use in color television receiver displays, the penetration phosphor color tube and the principles it employs offer several advantages for use in information displays. The penetration phosphor color display of the present invention overcomes the defects of prior art color displays, again permitting much more information to be displayed effectively simultaneously than by a black and white tube. Additionally, recognition of individual data representations in the displayed material when colored is much more rapid than in the monochromatic or black and white display.
The conventional penetration phosphor cathode ray tube in its most prevalent form has a viewing screen which uses controlled penetration of electrons into a series of phosphor layers for producing therein light of a corresponding series of colors. For example, the screen may include two different phosphor layers, each very thin and uniform, and separated by a transparent dielectric layer. When the electron beam is driven by a relatively low voltage, the energy of the electrons is not sufficient to penetrate the dielectric barrier layer; only the phosphor of the first layer is excited and only its corresponding color is produced. At higher electron beam voltages and correspondingly higher electron energies, the phosphors of both layers are excited; the intensity of the color contribution by the second layer phosphor increases as the electron beam energy is increased.
Full use may be made of the penetration phosphor characteristics in providing a variable color display while using only one electron beam simply by controlling the voltage on the beam acceleration electrode adjacent the cathode. With red and green emitting phosphors, successive changes in the beam acceleration voltage generate distinct colors; for example, red, orange, yellow, or green may thus be generated. Only one suitably controlled electron gun is required and the apertured masks of prior art color television tubes are eliminated along with other features undesirable for use in information displays.
The prior art provides penetration phosphor display tubes which appear to fall into one of four categories, the principal of these being the aforementioned multi-layer type in which several alternating layers of phosphors and dielectric barrier layers are placed on the screen of the tube by sedimentation. Very thin uniform layers are required for use with conveniently low acceleration voltages and they are difficult to form because of the many steps involved and of the close tolerances to be held on layer thicknesses. It has also been proposed to coat one or more layers of very small phosphor particles on larger phosphor particles emitting a different light color, the composite particles being built up using gelatin to hold them together. Thus, each particle is a multi-layered structure and may include alternated barrier layers. Particle size is a serious problem since the greater the number of layers added, the greater becomes particle size. The very small particles are not easily obtained and generally have low luminosity and are inefficient.
A third prior art approach involves the use of a barrier layer green phosphor mixed with a red phosphor. The barrier layers are formed on ZnS as a host material, so that only broad band green emission is obtained and not the desired line spectral emission. The green phosphor is coated with ZnO or ZnS or CoS is diffused into the surface of the ZnS. The green light produced by this structure is unsaturated and broadband and the colors are not distinct.
While certain rare earth materials have been proposed for use in display devices, the prior art has given only minor attention to rare earth materials for application in penetration phosphor displays. It appears that one approach to the penetration phosphor display area has been made using a rare earth material, but the color at low beam voltages is green and at high voltages, red is obtained. Such a particle cannot be mixed with an additional phosphor, to produce a phosphor which changes in color from red to green with increasing voltages since the first emission color is green. The saturation of the colors is inferior and color control is difficult since the same activator must be used for all light emission.
In Applicant's copending application Ser. No. 505,710, a penetration phosphor of the general type described herein is disclosed. In particular, the penetration phosphor therein consists of a core or central region comprised of a host material of La.sub.2 O.sub.2 S with a uniform distribution of an activator ion such as terbium (Tb) therethrough. The core or central region is surrounded by a barrier or peripheral region comprised of La.sub.2 O.sub.2 S with a distribution of co-activator ions therethrough; particularly Tb and Eu. This penetration phosphor yields a red spectral emission at low electron beam excitation voltage and a green spectral emission for high electron voltage excitation with the appropriate colors (orange and yellow) therebetween.